One example of positive electrode active materials for use in non-aqueous electrolyte secondary batteries is a lithium nickel composite oxide which includes nickel as the main component. A representative lithium nickel composite oxide is LiNiO2. It is known that lithium nickel composite oxides are, for example, in the form of monodisperse primary particles which do not form secondary particles or in the form of secondary particles comprising agglomerated primary particles. In either form, the mean particle size is as small as 1 μm or less, so the specific surface area is large. When a lithium nickel composite oxide in the form of primary particles is used as a positive electrode active material of a non-aqueous electrolyte secondary battery, the insertion and extraction of lithium is easy since the area of the positive electrode active material in contact with electrolyte (or liquid electrolyte) is large.
A lithium nickel composite oxide is prepared, for example, by mixing lithium hydroxide and a nickel hydroxide, and baking the mixture at a temperature of approximately 600 to 800° C. in an oxidizing atmosphere. Since the lithium nickel composite oxide prepared by this method has many void spaces therein, it has a problem of pressing characteristics in electrode production, so that the packing density of the lithium nickel composite oxide in the active material layer becomes low. As a result, the battery capacity tends to become low. This possibility is remarkably high particularly when a spherical nickel hydroxide is used.
Also, a lithium nickel composite hydroxide in the form of monodisperse primary particles having a particle size of less than 1 μm does not have void spaces therein, but the total volume of void spaces between the primary particles per unit volume is large. Hence, in the same manner as described above, a problem of pressing characteristics in electrode production (hereinafter “electrode pressing characteristics”) is likely to occur. Further, a lithium nickel composite hydroxide in the form of monodisperse primary particles adversely affects the stability of electrolyte or liquid electrolyte during storage, causing the electrolyte or liquid electrolyte to deteriorate. As a result, the power characteristics of the battery may lower.
Thus, one approach to reducing the void spaces between the particles can be to use particles larger than the conventionally used particles to form agglomerated particles. However, such an approach has not been specifically proposed.
Also, a method of producing a lithium nickel composite oxide has been proposed in which plate-shaped nickel hydroxide and a lithium compound are mixed, the mixture is dry ground, and the ground mixture is baked at 600 to 1000° C. in an oxidizing atmosphere (for example, see Patent Document 1). In the plate-shaped nickel hydroxide, the average major axis diameter of the primary particles is 1 to 50 μm, the average thickness is 0.1 to 10 μm, and the BET specific surface area as determined by N2-BET method is 0.1 to 5 m2/g. Patent Document 1 can produce a lithium nickel composite oxide that is in the form of plate-shaped particles having a relatively large particle size. Also, the technique of Patent Document 1 is characterized in that such a lithium nickel composite oxide can be obtained in a temperature range higher than 800° C., which is the upper limit of conventional baking temperature. It has been found that the use of such a lithium nickel composite oxide allows an improvement to some extent in electrode pressing characteristics.
However, according to the technique of Patent Document 1, if plate-shaped nickel hydroxide having a high nickel content is used in an attempt to obtain a high capacity lithium nickel composite oxide, the resultant lithium nickel composite oxide has a specific capacity (capacity per unit weight) that is low as an active material. It is well known from presentations at academic meetings etc. that if the baking is carried out in a temperature range of higher than 800° C. in particular, the resultant lithium nickel composite oxide has a significantly low specific capacity. The use of an active material having a low specific capacity in a battery inevitably results in the battery having low power characteristics.
As described above, according to conventional techniques, it is very difficult to improve the electrode pressing characteristics while maintaining the power characteristics of a battery.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 11-1324